Power control and modulation of switched-mode power amplifiers with one or more stages

ABSTRACT

This invention controls and modulates switched-mode power amplifiers to enable the production of signals that include amplitude modulation (and possibly, but not necessarily, phase modulation), the average power of which may be controlled over a potentially wide range.

BACKGROUND

1. Field of the Invention

The present invention relates to power amplifiers, particularlyswitched-mode power amplifiers.

2. Description of Related Art

Switched-mode power amplifiers have demonstrated the capability ofproducing, with high power-added efficiency (PAE), phase-modulatedsignals that have very high signal quality—i.e., low root-mean-square(RMS) phase error relative to an ideal signal and little or nodegradation in power spectral density (PSD). These power amplifiers havealso been demonstrated to be highly tolerant of temperature variation,and are believed to be highly tolerant to fabrication-process variation,making them attractive for high-volume applications such as consumerelectronics. Such power amplifiers include a switch connected to aresonant network; the output of the resonant network is connected inturn to a load (e.g., the antenna in a radio transmitter).

An early switched-mode amplifier is described in U.S. Pat. No. 3,900,823to Sokal et al., incorporated herein by reference. Sokal et al.describes the problem (created by unavoidable feedthrough from amplifierinput to amplifier output) of power control at low power levels andproposes solving the problem by controlling RF input drive magnitude toa final amplifier stage. In particular, the input drive magnitude of thefinal stage is controlled by using negative feedback techniques tocontrol the DC power supply of one or more stages preceding the finalstage. Various other known techniques use variation of amplifier powersupply for linearization as described, for example, in the followingpatents, incorporated herein by reference: U.S. Pat. No. 5,091,919; U.S.Pat. No. 5,142,240, and U.S. Pat. No. 5,745,526.

Another type of switched-mode amplifier, that does not require the useof negative eedback as in Sokal, is described in U.S. patent applicationSer. Nos. 09/247,095 and 09/247,097 of the present assignee, entitledHIGH-EFFICIENCY MODULATING RF AMPLIFIER and HIGH-EFFICIENCY AMPLIFIEROUTPUT LEVEL AND BURST CONTROL, respectively, filed Feb. 9, 1999(WO0048306 and WO0048307) and U.S. patent application Ser. No. ______(Dkt. 090729HEM2.US), entitled HIGH-EFFICIENCY MODULATING RF AMPLIFIER,filed Aug. 10, 2000, all incorporated herein by reference. In the latterswitched-mode power amplifiers, the average power is determined by twosignals: the switch supply signal and the switch control signal. Theswitch supply signal is the DC voltage available on one side of theswitch; as this voltage increases, the peak voltage of the oscillatorysignals developed within the resonant network and subsequently deliveredto the load also increases. The switch control signal is typically aphase-modulated signal that controls the switch (i.e., determineswhether the switch is on or off). This switch control signal should bestrong enough to toggle the switch on and off but should not beexcessively strong: unlike a linear amplifier in which the strength ofthe output signal is determined by the strength of the input signal, ina switched-mode power amplifier, if the switch control signal is toostrong, the excess signal merely leaks through the switch and into theresonant network (i.e., feedthrough). When this occurs, a version of theswitch control signal that is out-of-phase with respect to the desiredsignal adds to the desired signal within the resonant network, alteringboth the phase and the amplitude of the output signal in an undesirableway.

French Patent 2,768,574 also describes a switched-mode power amplifierarrangement. Referring to FIG. 1, in this arrangement, the poweramplifier circuit comprises a DC-to-DC converter 20 and a poweramplifier 30. The DC-to-DC converter 20 includes a pulse-width modulator22, a commutator/rectifier 24 and a filter 26.

The pulse-width modulator 22 is coupled to receive a DC-to-DC commandinput signal from a signal input terminal 21, and is arranged to apply apulse-width-modulated signal to the commutator/rectifier 24. Thecommutator/rectifier 24 is coupled to receive a DC-to-DC power supplyinput signal from a signal input terminal 25, and is also coupled toapply a switched signal to filter 26. The filter 26 in turn applies afiltered switched signal 28 in common to multiple stages of the poweramplifier 30.

A circuit of the foregoing type is substantially limited by thefrequency of the pulse-width modulator. In addition, common control ofmultiple power amplifier stages in the manner described may provedisadvantageous as described more fully hereinafter.

It is desirable to achieve more precise control ofswitched-mode-generated RF signals, including amplitude-modulatedsignals, such that the aforementioned benefits of switched-mode poweramplifiers may be more fully realized.

SUMMARY OF THE INVENTION

This invention controls and modulates switched-mode power amplifiers toenable the production of signals that include amplitude modulation (andpossibly, but not necessarily, phase modulation), the average power ofwhich may be controlled over a potentially wide range.

In order to produce amplitude-modulated signals, the DC switch supplyvoltage is replaced by a time-varying switch supply signal that isrelated to the desired amplitude modulation. This switch supply signalcan be either the desired amplitude modulation signal itself or apre-distorted version thereof, where the pre-distortion is such that theoutput signal has the desired amplitude modulation. In the latter case,the pre-distortion corrects for amplitude non-linearity (so-called AM/AMdistortion) in the switch and/or the resonant network.

The foregoing modification alone, however, may be insufficient toprovide as much dynamic range in the output signal as may be desired.Also, the modification may not be sufficient to maintain dynamic rangein the amplitude modulation while adjusting the average power of theoutput signal. Both of these problems are caused by the undesirableleakage signal described previously; its contribution to the output islargely independent of the level of the switch supply signal. That is,the switch supply signal may be reduced to zero volts (the minimumpossible amplitude), yet the output signal will still be at a relativelyhigh level; below some point, the amplitude modulation imparted throughthe switch supply signal is manifest less and less in the output signal.

Similarly, the severity of amplitude-dependent phase shift (so-calledAM/PM distortion) increases as the switch supply signal decreases. Thiseffect arises because the leakage of the switch control signal is out ofphase relative to the desired signal. As the switch supply signaldecreases, the desired signal decreases as well, whereas the leakagesignal does not; since these two signals are out of phase, the phase oftheir sum is increasingly dominated by the phase of the leakage signal.This invention, in one aspect thereof, modifies the switched-mode poweramplifier by adjusting the amplitude of the switch control signal toreduce the undesirable leakage effect. As a result, it becomes possibleto produce output signals having average power anywhere within a widerange, or to greatly increase the dynamic range over which amplitudemodulation may be produced at a given average power level, or both.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The present invention may be further understood from the followingdescription in conjunction with the appended drawing figures. In thefigures:

FIG. 1 is a block diagram of a known switched-mode power amplifier in avariable power supply voltage is applied in common to multiple stages;

FIG. 2 is a block diagram of a switched-mode power amplifier withoutamplitude modulation capability;

FIG. 3 is a diagram comparing AM/PM distortion in a switched-mode poweramplifier without a countermeasure of the invention and with acountermeasure of the invention;

FIG. 4 is a waveform diagram of waveforms in the circuit of FIG. 2;

FIG. 5 is one possible circuit that may be used to control theapplication of power to one or more power amplifier stages;

FIG. 6 is another possible circuit that may be used to control theapplication of power to one or more power amplifier stages;

FIG. 7 is still another possible circuit that may be used to control theapplication of power to one or more power amplifier stages;

FIG. 8 is a block diagram of a generalized efficient power amplifierstructure;

FIG. 9 is a block diagram of a switched-mode power amplifier havingamplitude modulation capability;

FIG. 10 is a waveform diagram of waveforms in the circuit of FIG. 9;

FIG. 11 is another waveform diagram of waveforms in the circuit of FIG.9;

FIG. 12 is a more detailed diagram of an exemplary embodiment of theswitched-mode power amplifier of FIG. 9; and

FIG. 13 is a waveform diagram of waveforms in the circuit of FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2, a block diagram is shown of a switched-modepower amplifier. A switch 201 is coupled to a resonant network 205 andto power control logic 215, which is coupled in turn to a DC supply 203.The resonant network is coupled to a load 207. Control of the switch 201is accomplished using a control signal 209, applied to an amplifier 211.The amplifier 211 produces a switch control signal 219, which is appliedto the switch 201. As the switch 201 is opened and closed responsive tothe control signal 209, the resonant network 205 shapes the switchvoltage to produce a desired output signal 213.

In the amplifier of FIG. 2, the signals 209 and 219 areconstant-amplitude (CA) signals (i.e., oscillatory signals having aconstant peak amplitude) that may be phase-modulated. The amplitude ofthe switch control signal 219 is set by the power control logic 215. Thepower control logic 215 also controls a DC supply voltage 216 producedby the DC supply 203 and supplied to the switch 201. As the powercontrol logic 215 causes the DC supply voltage 216 to increase, the peakvoltage of the oscillatory signals developed within the resonant network205 and subsequently delivered to the load 207 also increases.Similarly, as the power control logic 215 causes the DC supply voltage216 to decrease, the peak voltage of the oscillatory signals developedwithin the resonant network 205 and subsequently delivered to the load207 also decreases.

Further details of the amplifier chain of FIG. 2 in accordance with anexemplary embodiment of the invention are described in the foregoingcopending U.S. patent applications. In addition, a bias controlarrangement may be used to achieve optimal bias of the switch 201 undervarious conditions as described more fully in U.S. patent applicationSer. No. ______ (Dkt. 101006VBC.US), filed on even date herewith andincorporated herein by reference.

In accordance with one aspect of the invention, a signal 218 is used tocontrol the amplitude of the switch control signal 219 in a coordinatedmanner with control of the DC supply voltage 216, thereby avoidingexcess leakage of the switch control signal 219 through the switch 201and into the resonant network 205.

More particularly, in any physical embodiment, a stray (unintended)capacitance 212 around the switch 201 is unavoidably present. This straycapacitance provides a leakage path for the switch control signal 219 toleak into the resonant network 205, where it mixes with the desiredswitch output signal. Since the switch control signal 219 isout-of-phase with the desired switch output signal, a large phase shiftwill occur at the switch output when the desired output signal magnitudeis near to or smaller than that of the leakage signal. This effect isshown in FIG. 3, which depicts output phase and output magnitude asparametric functions of desired magnitude (i.e., as desired magnitudedecreases, the curves of FIG. 3 are traced out in the counter-clockwisedirection). In the illustrated case, signal leakage is assumed to be 35dB below the maximum output signal (1.7%), at a relative phase shift of−170 degrees. If the switch control signal is not reduced (line A), thenthe amplifier output signal suffers severe AM-PM (and AM-AM) distortionwhen the desired output magnitude is less than 10% of the peak outputmagnitude.

This effect may be counteracted, for lower amplitude output signals(e.g., less than 10% of the peak output magnitude), by correspondinglyreducing the switch control signal (e.g., to 10% of its original value).As FIG. 3 shows, this measure essentially removes the AM-PM and AM-AMdistortion from the desired output signal (line B). In principle, thistechnique can be extended to arbitrarily low desired output signalmagnitudes.

For illustration purposes, consider the need to produce aconstant-amplitude RF signal in a time-slotted network, in which theoutput power may vary from slot to slot. In the amplifier of FIG. 2,this manner of operation may be achieved by holding the supply voltage216 constant during a given time slot, and by holding the peak amplitudeof the control signal constant during the time slot as illustrated inFIG. 4. As a result, the peak amplitude of the output signal 213 isconstant during a given time slot. Note that when the supply voltage 216is is at a low level, the control signal 219 is also at acorrespondingly low level (e.g., time slot (N)). In this manner, thelow-distortion characteristic of line B of FIG. 3 is achieved.

Various specific circuits that may be used within the power controllogic 215 of FIG. 2 to control the application of power to the amplifierstages are shown in FIG. 5, FIG. 6, and FIG. 7, respectively.

Referring first to FIG. 5, a DC supply voltage V_(SUPPLY) is applied tothe emitter of a PNP bipolar transistor Q in common-emitterconfiguration. The DC supply voltage may be unregulated or,alternatively, may have been regulated/conditioned to an appropriate DClevel for a desired instantaneous output power using, for example, aswitching power supply in combination with a linear regulator asdescribed in greater detail in the aforementioned patent applications.The collector of the transistor Q is connected through a resistivedivider network R1, R2 to ground. An operational amplifier 501 isconnected to receive a power-setting command signal 523 on a negativeinput and to receive on its positive input a voltage developed at thejunction of the resistors R1 and R2. The operational amplifier 501produces an output signal that is applied to the base of the transistorQ. In operation, the transistor functions as a controlled resistance,under control of the operational amplifier 501, to deliver aprecisely-controlled voltage to multiple amplifier stages, including,for example, a driver stage 503 (responsive to an RF signal 509analogous to signal 209 of FIG. 2) and a final stage 505. In the case ofthe driver stage 503, the controlled voltage from the transistor Q isapplied through a resistor R3 to account for the sizing of the driveramplifier relative to the final amplifier. The foregoing circuitrealizes fast control and may be used in conjunction with or in lieu ofseparate DC regulation circuitry.

One or more additional driver stages may be provided as shown, forexample, in FIG. 6. In FIG. 6, the supply voltage of an initial stage607 is controlled less stringently. A number of discrete supply voltages(V₁, V₂, . . . , V_(N)) are applied to a switch 609, which is controlledto select a desired one of the discrete voltages. Control of the finalstage 605 and the immediately preceding driver stage 603 may remain aspreviously described.

If a desired output signal has a large dynamic range, common control ofthe driver and final stages may prove insufficient. Referring to FIG. 7,separate control is provided for each of multiple amplifier stages. Thisarrangement may be extended to any arbitrary number of stages.

Referring again to FIG. 2, in the case of constant amplitude outputsignals, the amplifier as shown is effective to provide efficientamplification and power control. However, it does not provide foramplitude modulation.

Referring now to FIG. 8, a generalized efficient power amplifierstructure is shown, enabling control of multiple stages to achievecomplex control, including amplitude modulation, of an amplifier outputsignal. In FIG. 8, an RF input signal, RF_(in), is applied to anamplifier chain including N stages. The amplifier chain produces an RFoutput signal, RF_(out). Supply voltages for each of the stages areindependently controlled. One or more control blocks receive a DC supplyvoltage and, responsive to control signals from a controller (notshown), produce separate power supply voltages for each of the Namplifier stages. In the example of FIG. 8, two control blocks areshown, a power/burst control block 801 and a modulation control block803. However, the functions of the control blocks may be readilyconsolidated or sub-divided as will be apparent to one of ordinary skillin the art.

Optionally, independent bias signals may be supplied to each one of thestages. In one embodiment, possible values of the bias signal include avalue that turns the stage off, e.g., places the active element of thestage in a high-impedance state. In addition, each stage may optionallyinclude a controlled bypass element or network, shown in FIG. 8 as aresistor connecting the input and output terminals of a stage. Such abypass may allow performance of an amplifier stage at low input signallevels to be more completely characterized and controlled. Inparticular, since circuit parasitics unavoidably create the effect of abypass, by explicitly providing a bypass, it may be designed in such amanner as to dominate parasitic effects.

A particular case of the generalized amplifier structure of FIG. 8 willnow be described in detail.

Referring to FIG. 9, an amplifier is shown that provides the advantagesof the amplifier of FIG. 2 and additionally provides for amplitudemodulation. In FIG. 9, there is provided a switch 901, a DC supply 903,a resonant network 905, a load 907, a control signal 909, a controlsignal amplifier 911, an output signal 913 and power control logic 915,corresponding generally to and given like designations as elements inFIG. 2. The control signal amplifier 911 is responsive to a drivecontrol signal 918 to produce a switch control signal 919 In FIG. 9,however, there is additionally provided an amplitude modulator 917responsive to an AM signal 923. Instead of the power control logic 915controlling the control signal amplifier 911 directly (as in FIG. 2),the power control logic 915 is coupled to the amplitude modulator 917,which is responsive to the power control logic 915 to control thecontrol signal amplifier 911. Under the control of the amplitudemodulator 917, the control signal amplifier 911 produces a switchcontrol signal 919 that is applied to the switch 901. The DC supply 903is coupled to the amplitude modulator 917, which is responsive to the AMsignal 923 to modify the supply voltage appropriately and apply aresulting switch supply signal 921 to the switch 901.

Two cases of operation of the amplifier of FIG. 9 may be distinguished.One case is shown in FIG. 10, in which amplitude modulation is achievedsolely through variation of the switch supply signal 921, and powercontrol is achieved jointly through variation of the DC supply 903 andvariation of the switch control signal 919 (via signal 918). During atimeslot (N−1), the peak amplitude of the switch control signal 919remains constant. During this time, the peak amplitude of the controlsignal 909 also remains constant. The switch supply signal 921, on theother hand, has impressed upon it amplitude modulation signalvariations. As a result, the output signal 913 exhibits correspondingamplitude variations. During timeslot (N), the amplitudes of the controlsignal 909 and the switch control signal 919 are constant at a lowerlevel, and a DC supply voltage 904 (not shown in FIG. 10) is alsoconstant at a lower level, indicative of a lower desired output powerlevel. Different amplitude modulation signal variations are impressedupon the switch supply signal 921 and are manifest in the amplitude ofthe output signal 913. During timeslot (N+1), the level of the controlsignal 909 and the switch control signal 919 are raised back up, as isthe DC supply voltage 904, corresponding to a higher desired outputpower level. The constant peak amplitude of the switch control signal919 is set higher for higher desired output power levels, and set lowerfor lower desired output power levels, so that the switch 901 issuccessfully turned on and off as needed while minimizing theundesirable leakage of the switch control signal 919 through the switch901 and into the resonant network 905.

At lower power levels, to avoid excess leakage of the switch controlsignal 919 into the output signal 913, it may be necessary to achieveamplitude modulation of the output signal through coordinated variationof both the switch supply signal 921 and the switch control signal 919.This represents the second case of operation previously referred to, andis illustrated in FIG. 11. In particular, FIG. 11 shows examples ofdifferent relationships between amplitude modulation of the switchsupply signal 921 and amplitude modulation of the switch control signal919. Power control and amplitude modulation of both the switch supplysignal 921 and the switch control signal 919 are applied as needed toextend the dynamic range of the output signal 913. In an exemplaryembodiment, amplitude modulation of the switch control signal 919 isapplied only when the AM signal 923 dips below a threshold that ispower-level dependent.

Timeslot (N−1) illustrates the case in which the AM signal 923 is belowthe power-level-dependent threshold (indicated in dashed lines in theupper frame of the FIG. 11) for the duration of the timeslot. Hence, theswitch control signal 919 is amplitude modulated along with the switchsupply signal 921 throughout the duration of the timeslot. In timeslot(N), during both an initial portion of the timeslot and during a finalportion of the timeslot, the AM signal 923 is assumed to be above thethreshold. Hence, during these portions of the timeslot, the switchcontrol signal 919 is not amplitude modulated. (In the middle frame ofFIG. 11, the dashed lines indicate the nominal amplitude of the switchcontrol signal 919 when the AM signal 923 is above the threshold.)During an intermediate portion of the timeslot, however, the AM signal923 is assumed to be below the threshold. During this portion of thetimeslot, the switch control signal 919 is amplitude modulated alongwith the switch supply signal 921. Finally, in timeslot (N+1), the AMsignal 923 is assumed to be above the threshold throughout the durationof the timeslot. The amplitude (peak-to-peak) of the switch controlsignal 919 is therefore held constant throughout the duration of thetimeslot. Note that the actual amplitude modulation is still solelyimpressed on the output signal 913 by switch supply signal 921.Variation of signal 918 and the resulting variation of signal 919 inconcert with signal 921 is performed soley to reduce leakage. As such,the precision required of signal 918 is greatly reduced from thatrequired of signal 921.

Referring now to FIG. 12, a more detailed diagram is shown of anamplifier in accordance with an exemplary embodiment of the invention,in which like elements are assigned like reference numerals as in FIG.9. In the embodiment of FIG. 12, the control signal amplifier 1211 andthe switch 1201 are provided as first and second amplifier stages, a“gain” stage and a “switch” stage, respectively. The gain stage 211 maybe implemented in a variety of ways. One implementation is aconventional gain-controlled linear CCS (controlled current source)amplifier of widely-understood classes A, AB, B and C. An alternativeimplementation is a smaller-scale switch-mode stage of a type describedin the aforementioned copending U.S. applications.

Within dashed line block 917 are shown further details of one embodimentof the amplitude modulator 917 of FIG. 9. In response to AM signalsamples 1223 and to a signal 1232 from the power control logic 1215, theAM logic 1231 calculates appropriate supply levels for the firstamplifier stage 1211 and the second amplifier stage 1201.

In the case of the first amplifier stage 1211, a DC supply voltage issupplied through a transistor 1235-1. Base drive to the transistor1235-1 is controlled by the AM logic 1231 through a DAC (digital toanalog converter) 1233-1. Hence the DAC 1233-1 sets the level of theswitch control signal 1219 seen by the second amplifier stage 1201.Similarly, in the case of the second amplifier stage 1201, a DC supplyvoltage is supplied through a transistor 1235-2. Base drive to thetransistor 1235-2 is controlled by the AM logic 1231 through a DAC1233-2.

In an exemplary embodiment, the output of the DAC 1233-1 is given by thefollowing rule: $\begin{matrix}{{{{DAC}_{1}(t)} = {v(p)}},} & {{{for}\quad{a(t)}} \geq {m(p)}} \\{{= {{v(p)} \cdot \frac{a(t)}{m(p)}}},} & {{{for}\quad{a(t)}} < {m(p)}}\end{matrix}$where a(t) is the AM signal at time t, m(p) is a threshold dependent onthe power level p, and v(p) is the nominal output voltage of DAC₁, forpower level p.

Operation of the amplifier of FIG. 12 in accordance with the foregoingrule is illustrated in FIG. 13. As seen therein, as the signal a(t) (theamplitude of the AM signal at time t) fluctuates, for a first period oftime, the signal exceeds the threshold m(p) for the current power levelp. During this period, the voltage DAC₁(t) is set to the nominal levelv(p). Thereafter, the signal a(t) dips below the threshold for a periodof time. During this period of time, the voltage DAC₁(t) is amplitudemodulated in accordance with the fluctuations of the signal a(t). Whenthe signal a(t) again rises above the threshold, the voltage DAC₁(t) isagain set to the nominal level.

Thus, there has been described an efficient amplifier for RF signalsthat provides for amplitude modulation over a wide dynamic range. Theamplitude of the switch control signal is adjusted to reduce theundesirable leakage effect. As a result, it becomes possible to produceoutput signals having average power anywhere within a wide-range, or togreatly increase the dynamic range over which amplitude modulation maybe produced at a given average power level, or both.

It will be apparent to those of ordinary skill in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential character thereof. The describedembodiments are therefore intended to be in all respects illustrativeand not restrictive. The scope of the invention is indicated by theappended claims, rather than the foregoing description, and all changeswhich come within the meaning and range of equivalents thereof areintended to be embraced therein.

1-85. (canceled)
 86. An amplifier apparatus, comprising: a driver havingan RF input port configured to receive an RF input signal, a drivercontrol port configured to receive a driver control signal, and driveroutput port configured to provide a drive signal having a constant, butadjustable, peak amplitude value; an amplifier stage having an RF inputport configured to receive the drive signal, a power supply portconfigured to receive an adjustable supply signal, and an RF output portconfigured to provide an output RF signal; and a control circuitoperable to dynamically adjust the driver control signal and theadjustable supply signal in a manner that reduces distortion in the RFoutput signal.